These investigations are concerned with the role of actin and several associated proteins in the motile processes of cell division and early development, using as experimental material the synchronously dividing eggs of the sea urchin. Warming of extracts of the cytoplasmic proteins of these eggs induces the polymerization of actin monomers to the filamentous form, and this F-actin then combines with two other proteins in the extract of molecular weights of 58,000 and 220,000 to form a gel with a characteristic banded structure at the electron microscope level. Optical diffraction studies on electron micrographs of this material allowed the construction of a model for this highly structured polymer and the individual proteins have recently been isolated and purified and found to spontaneously reform this unit when combined. Evidence from other studies suggest that this aggregate of actin and these other proteins exist in the cell as a mechanical skeleton for such organelles as microvilli and filopodia. Further studies are now under way on the relation of these actin-based structural units to the actin-myosin based contractility of cytoplasm, with particular reference to the process of cell division.